Continuous tone thermoplastic photography



Dec., 23, 1969 'wlLFR'l-H ET AL 3,485,623

cc NT11\xlJ sv ToNE'HERMoPLAsTIc PHOTGRAPHY Filed April 11, 196e 2Sheets-Sheet 1 455.2922... IAQ o2 A: .Emzwa mom- 19 vv O24 my Dec. 23,1969 R; A wlLF-"ERT". ET AL i 3,485,623

cONTNUoUs TONE. THERMOPLASTIC PHOTOGRAPHY Filed April 11. 196e :esheets-sheet 2 o"max 500 (2) vous v T 0R U 40 (l) oc (2) v0=vR=3oovam()l (3) O: (4) :Vo= VR= 400V (5)0( (6) =v=vR=5OOv (5) (3)v K (l) l lYl l I l l F/'6 4 RELATIVE-UNITS oF LOG EXPOSURE FROST DENSITY FROSTDENSITY x )n A ll LOG E (mcs) INVENTORS.

ROBERT A. wxLFERTH APETER s. KEI-:NAN

a@ gfllw ATTORNEY United States Patent 3,485,623 CONTINUOUS TONETHERMOPLASTIC PHOTOGRAPHY Robert A. Wilferth, Pittsford, and Peter B.Keenan, Rochester, N.Y., assignors to Xerox Corporation, Rochester,N.Y., a corporation of New York Filed Apr. 11, 1966, Ser. No. 541,835Int. Cl. G03g 13/00 U.S. Cl. 96-1.1 3 Claims ABSTRACT OF THE DISCLOSUREA process enabling control of the photo-response characteristics of afrostable member wherein the initial charging and recharging potentialsare altered so as to adjust the spread between that exposure whichresults in threshold frosting and that exposure which results in maximumfrosting until this exposure spread is in such ratio to the differencebetween image density present at threshold frosting and image densitypresent at maximum frosting as to produce the desired gamma in theparticular frostable member being utilized.

This invention relates generally to deformation imaging processes and,more specifically, to processes of the type identified as electrostaticfrosting.

In U.S. Patent No. 3,196,011 to Kenneth W. Gunther and Robert W.Gundlach, a new form of deformation imaging is disclosed in which anelectrostatic pattern is used for selectively frosting in imageconfiguration a continuous deformable lm or layer. The frost processestherein disclosed result in images having properties remarkably atvariance with those properties displayed by images produced by earlierknown deformation processes. More specically, and as is fully set forthin the Gunther and Gundlach patent alluded to, these earlier techniquesrelied upon fringing fields present at the edge of boundaries separatingcharged and uncharged areas on the deformable surface to produce thereona pattern of depressed and elevated areas corresponding to the chargedand uncharged areas, respectively, whereby a mere relief or outlinepattern resulted when one attempted to image an optical patternincluding continuous tone areas. The processes of electrostatic frostingon the other hand are such as to deform both solid and/ or line areas ofcharge on the mechanically deformable material into relatively orderlypatterns of microscopic discontinuities whereby gross light scatteringcharacteristics are present at all points on the deformation image wherean optical input was originally presented.

It is therefore apparent from the foregoing that the frost images towhich the present invention is applicable display continuous-tonereproduction capabilities much like conventional silver halide images.Clarification of the type of problem to which the present inventionaddresses itself may in fact be best gained by considering thesemianalogous silver halide images somewhat further and in particularnoting that one of the most important characteristics of such imagesderives from the fact that the relationship present therein betweenimage density and exposurewhich is to say the so-called gamma of thephotographic emulsion-may be controllably varied by developmentprocesses, and so forth, so as to augment or lessen the gammacharacteristics of the film. By virtue of such controllable variations,corresponding changes may be introduced in the contrast rangecharacteristics of the developed image; which is to say that the tonalgradations in a reproduced scene or the like may be adjusted to suit theeye of the viewer, to emphasize lowlight features, or so forth. Aspecific and appropriate ICC example of this manipulative techniquearises in the art of processing photo reconnaissance film or the likewhere it is well known to adjust both development and/or exposure of thepositive print so as to best utilize the gamma characteristics of theprinting stock to achieve the most desirable (usually meaninginformation-laden) final picture.

Now in accordance with the present invention we have discovered that thephotographic response properties of frostable photoreceptor members toomay be controlled by a process to be hereinafter set forth, whereby itbecomes possible to adjust not only the gamma characteristics, but tosome extent the speed as well of these materials, so that the resultingfrost image will display the characteristics and treatment ofinformation -content desired by a user.

It is thus an object of the present invention to provide a processwhereby the response characteristics of a frostable ,member may bevaried to suit the requirements imposed by the optical input theretoand/ or the subsequent use thereof intended by a user.

It4 is a further object of the present invention to provide a processwhereby the gamma characteristics of a frostable member may be adjustedto suit the requirements of a user.

It is an additional object of the present invention to provide a processwhereby a frost image corresponding to an optically imaged scene may beestablished with a contrast gradation range of determinative width.

It is yet an additional object of the present invention to provide aprocess whereby the speed of a frostable photoreceptor may bedeterminatively adjusted.

In the present invention these explicit objects, and other implicitobjects as will become apparent in the ensuing specification, arebrought about by a process wherein the initial charging potential andthe recharge potential utilized in the frosting process are so adjustedthat the curves representing the physically measurable quantity frostdensity as a function of light exposure are made to assume a desiredshape; more specifically these curves--representative of the physicalquantity frost density-are made to acquire a desired span width betweenthe points at which such curves represent a frost threshold value andthe points of maximum frosting. In purely physical terms this means thatthe initial charging and recharging potentials are determinativelyaltered so as to adjust the spread between that exposure which resultsin threshold frosting and that exposure which results in maximumfrosting until this exposure spread correlates with a desired exposurerange or gam-ma in the particular frostable member utilized.

' A full understanding of the manner in which the present inventiveprocess is practiced may now be gained from a reading of the followingdetailed specification and from a simultaneous examination of thedrawings appended hereto in which:

FIGURE l is a graphic representation of a three layer frostablephotoreceptor member of the type utilized in the present invention.

FIGURE 2 is a representative curve illustrating the relationship betweenfrost density and surface charge density achieved on a member of thetype depicted in FIGURE l.

FIGURES 3 and 4 are analytical curves depicting how in the practice ofthe present invention, surface potential and surface charge densities onmembers as in FIGURE l may be expected to vary with the log of exposure.

FIGURES 5 and 6 are empirical `curves depicting variations of the typeshown in FIGURES 3 and 4 for the exemplary member set forth in ExampleI.

In FIGURE 1 a typical frostable photoreceptor device is depicted,generally resembeling the frostable members described in connection withthe Gunther and Gundlach patent previously alluded to. This frostablemember 1 is seen to include a conductive substrate 2 convenientlycomprising a thin sheet of aluminum, a photoconductive layer at 4, andan overcoated frostable layer at 6. For present purposes, thephotoconductive layer may be considered to comprise a layer of vitreousselenium having a thickness designated as ds and a permittivitydesignated as es; however, in some instances this layer will actually becompound in nature and consist of a lower portion of selenium aloneintended to exercise a charge storage function, and an upper portioncomprising a relatively panchromatic selenium-tellurium alloy andintended to function as the photoconductor proper.

The upper most layer 6, the frostable component, is shown in FIGURE 1 ashaving a thickness dp and a permittivity designated as ep. A list ofmaterials suitable for this frostable component is indicated in theGunther and Gundlach patent previously mentioned. In a very typicalexample such layer will comprise a few microns thick coating ofStaybelite Ester-10 (glyceryl tri-ester of 50% hydrogenated wood resin)available under the trade name indicated from the Hercules PowderCompany, Wilmington, Del. It might be noted that in some instances aninterlayer comprising several hundredths of a micron of an organicmaterial may be applied between the photoconductive layer 4 and thelfrostable layer 6, as such interlayer appears t operate in some obscuremanner to increase resolution in the final frosted image. However, `forpurposes of the present invention, this interlayer need not bespecically considered or included and in any event may for purposes ofanalysis be thought of as included in the frostable layer 6.

In the practice of electrostatic frosting as taught in the Gunther andGundlach patent, the frostable member 1 is initially provided withcharge on the surface of layer y6 by moving the member relative to acorona charging device or the like. The member is then exposed to apattern of light and shadow as, for example, by means of a photographicenlarger or the like, which permits migration of charges in portions ofthe photoconductive layer 4 adjacent to light exposed areas on themember. These migrating charges become bound at the interface betweenlayers 4 and 6 with a resulting lowering of potential at correspondingpoints on the surface of the deformable layer 6. Subsequently, the layer6 is recharged by the same or a similar corona device so as to bring thesurface `6 once again to an equipotential. In areas of previousexposure--and thus of internal charge migration-the surface of 6 acceptsadditional charge, as a result of which the electric field and thepotential difference across such regions is also greatly increased. Uponsoftening of the deformable layer 6 those areas subjected to thisincreased electric iield frost to a degree that bears a relationship tothe eld or to the charge density present thereon.

In FIGURE 2. a representative curve is shown illustrating the typicalexperimental frost density achieved upon such a frostable member as afunction of the charge density present on the member after recharging.As used herein the term density refers to the light attenuation, 1.e.,

log I produced in an optical path including the frosted member; which isto say that density may -be regarded as a direct indication of lightscattered out of the optical path by the frost deformations. The curveshown is generally of the type shown in FIGURES 6A and 6B of the Guntherand Gundlach patent and is intended to illustrate firstly that in anyfrost process a certain minimum value of charge density -must beachieved before .4 any frosting takes place; and secondly that a certainmaximum value of charge density exists after which frost density showsno appreciable increase. These two critical values are depicted inFIGURE 2 by the designations 0T and Umax, respectively, the subscriptsbeing chosen to suggest the threshold and maximum useful values of thisparameter.

The essence of the present invention resides principally in thediscovery of a technique whereby the range of frost densities extendingbetween aT and Umax may be made to coincide with exposure ranges ofvarying widths. In the conventional parlance of photographic engineeringthis implies a method for varying the so-called gamma of thephotosensitive member, and this is in fact the result that is achievedby the present discovery. The discovery will further be seen in whatensues to indicate a method for adjusting the sensitivity of thephotoresponsive member-at least to a reasonable degree.

An understanding of the precise method by which the present invention ispracticed may now best be gained by a step-by-step analysis of themanner in which the density of charge present on a frostable memberafter recharge URE may be expected to vary in a generalized frostprocess including the possibility of differing values in the initial andrecharge potentials utilized.

es fp es In terms of surface potential the effect of dark decay cannotbe distinguished from effects of exposure; therefore, the change insurface potential AV includes exposure and dark decay. The charge at theinterface UE includes the charge due to exposure and dark decay. Hence,

VE=U0 da The photoreceptor member 1 is then recharged to restore thecondition of an equipotential surface. Charging to a potential VR(VR V0)will result in a surface charge density URE in the exposed area and RDin an unexposed area. Hence, in the dark R RD el en and in the exposedarea magg-ag) l D B By combining the values of AV and VR the surfacecharge density in the exposed area is found to be This analysis showsthat the final surface charge density is a function of--among otherthingsthe two independent processing parameters representd by theinitial potential V0 and the recharge potential VR, as well as of theexposure.

FIGURE 3 illustrates a representative set of curves that result onutilizing Equation 1 to determine surface charge density URE as afunction of log exposure for a representative device constructed inaccord with FIGURE 1. Variation in surface potential, VE as a result ofexposure and before recharge is also shown on the same set of axes.Exposure units in FIGURE 3 may be assumed to be relative, absolutevalues not being of any particular relevance to the present discussion.

Curves 1 and 2 in FIGURE 3 show the values for initial and rechargepotentials equal to 400 volts each. Curves 3 and 4 show the values foran initial potential of 200 volts and recharge potential of 550 volts.It may be noted in these curves that, as would be expected, the chargedensity curves at 2 and 4 are mirror images of the potential curves at 1and 3. It will also be noted as a general feature that curves 1 and 2join at the vertical axis whereas 3 and 4 are separated by a degree thatresults from the voltage differential between the initial potential andthe recharge potential utilized.

Also shown on the same set of axis are the charge density threshold mrand the charge density rmx to yield maximum density. We note here thatthe intersection of curves 2 and 4 with aT line gives the exposure toproduce a threshold value of frost density. Furthermore, theintersection of curves 2 and 4 with the cmax line gives the exposure toproduce a maximum frost density. We therefore see that in the case ofthe processing parameters utilized in connection with curve 4, theuseful exposure range is represented by the spread at 10 whereas in thecase of processing parameters in accord with curve 2 the useful exposurerange has narrowedand has also been displaced-to the spread at 12.

The average slope of curves 2 and 4 between 1T and max multiplied by theslope of the curve relating r and frost density D, is approximately thegamma. As is clearly apparent from the ligure the gamma value obtainableunder conditions producing curve 2 differs markedly from that obtainableunder conditions producing curve 4.

In addition it may be noted that the displacement of the exposure rangeto the left as in curve 4 has resulted in increasing the effectivesensitivity of the frostable member in that threshold frosting occurs inthis case at a much lower exposure than would be the situation wherecurve 2 were applicable, This factor is more strikingly illustrated inFIGURE 4 where curves are depicted showing surface charge density andsurface potential as a function of log exposure for instance whereas therecharge and initial charging potentials are in each curve set equal,but are made successively higher in adjacent sets of curves. Thus, inthat ligure curves 1 and 2 illustrate the effect where V: VR=300 volts,curves 3 and 4 where and in curves 5 and 6 where V0: VR=50O volts. Ineach instance the slopes of adjacent curves are roughly equal; however,the speed or sensitivity of the particular frostable member to whichthese processing parameters are applied increases from curve 2 to curve4 to curve 6. In fact, curve 6 actually shows VR exceeding the thresholdvalue for frosting even in the absence of exposure implying a backgroundlevel of fog and the highest speed of the lot.

The method taught by the present inventive process should by this pointbe quite clear. Essentially one utilizes in a frost process charging andrecharging parameters in accord with the gamma response desired. So, forexample, if one desires relatively flat response characteristics,processing parameters resembling those associated with curve 3 in FIGURE3 can be utilized by charging prior to exposure to a relatively lowinitial potential, and subsequently frosting over the resultingrelatively fiat-response regions utilizing a higher rechargevoltage-sufficient to elevate the flat portion of the charge densitycurve into the desired exposure range as at 10. On the other hand, onemay achieve the short spread high gamma exposure as in, for example,curves 1 and 2 by utilizing relatively high initial potentials andsuitable recharge potentials to properly position the exposure range asat 12.

It will, of course, be appreciated by those skilled in the art that theapproach to describing the present invention has thus for beencompletely general and is accordingly in no way limited to particularfrostable members possessing particular layer thicknesses,permittivities, or so forth. Thus, in particular, curves such as appearin FIG- URES 3 and 4 may be readily constructed for any given frostablemember. If sufficient data is available this can be done analytically;however, as a practical matter, it is more likely that with a givenstructure these curves will initially be determined empirically--that isby direct experimentation. However, once such curves are determined,whatever the process may be, they thereafter may be utilized inprecisely the manner that has been indicated since the determinativephysical processes that underlie establishment of these curves arerepeatable and not subject to unknown variations.

An illustration of the manner in which such empirical curves may beestablished and utilized will now be set forth:

EXAMPLE A frostable photoreceptor member essentially in struc- 'turalaccord with that depicted in FIGURE 1 was prepared wherein the substrate2 comprised an anodized aluminum sheet 0.05 inch thick. The layer in theinstant example was compound and included a selenium charge storagelayer 25 microns thick adjacent the conductive substrate. Upon theselenium layer was coated a .3 micron thick photosensitive layercomprising an alloy having a nominal composition of 75% selenium and 25%tellurium, the latter layer by virtue of its composition exhibitinggenerally panchromatic light response. In the present example aninterlayer of an organic material about 0.05 micron thick was appliedover the photosensitive selenium tellurium layer, its function being toobtain the maximum possible resolution with the device. The mechanism ofoperation of this interlayer is not clear.

top layer was coated over the interlayer as the thermoplastic frostablelayer corresponding to 6 in FIGURE 1. In the present example, this layercomprised about 2 microns of Staybelite Ester-10 (glyceryl tri-ester of50% hydrogenated wood resin). This particular material is availableunder the trade name indicated from the Hercules Powder Company,Wilmington, Del. Storage layers and photosensitive layers were appliedby vacuum deposition. The interlayer and the frostable layer wereapplied by dip coating from a solution.

Experimental data was obtained of the variation of frost density D withlog E for this frostable member. Two such experimental curves are shownin FIGURES 5 and 6, the rst depicting the D-log E curve for theprocessing condition V0=400 volts, VR=400 volts, the second indicatingthe curve for the processing condition Vozvolts, VR=600 volts. Table Ibelow shows a compilation of data on this member for various othercombinations of initial and `iinal potentials. It will be noted that notonly has variation in gamma been achieved in accordance with theanalysis previously made but the speed or sensitivity of the member hasalso been varied in accord with such analysis.

TABLE I Final Speed Initial potential potential [L8/Ems Gamma Dm" In theforegoing data speed is intended to imply an ASA type of definition andis specifically defined as 0.8 divided by the exposure metercandleseconds to produce a ydensity 0.1 unit above base plus log. Toobtain the gamma a line is drawn from a point on the D-log E curve 0.1density unit above base plus log to a similar point 7V 0.1 density unitbelow Dmax. The slope of this line is taken as gamma.

Having once determined the gamma and/or speed of a frostable memberunder given processing conditions, we may thereafter set out processingconditions so as to utilize this frostable member in accordance with ournee'is. This is to say that we may thereafter decide what gamma responseor sensitivity is desired in a given exposure situation, and byreferring to the previously determined gamma and sensitivity vs. voltagecurves readily determine the proper processing parameters--viz., theproper charging and recharging voltages-appropriate to achieve thedesired results.

Having thus described the present inventive process it will be apparentthat those skilled in the art may now readily devise numerous variationsthereupon and deviations therefrom that will yet fall Within theprovince of the present invention. Accordingly, the invention is to bebroadly construed and limited only by the spirit and scope of the claimsappended hereto.

What is claimed is:

1. A method for adjusting the gamma response in a frostablephotoreceptor member including a conductive substrate overcoated with aphotoconductive layer of thickness ds and permittivity es in turnovercoated with a deformable frostable thermoplastic layer of thicknessdp and permittivity ep comprising:

(a) charging the surface of said member to a rst potential V;

(.b) exposing said -member to a light pattern;

(c) recharging said member to a recharge potential VR where the absolutevalue of VR is greater than the absolute lvalue of V0;

(d) and softening said frostable thermoplastic layer so as to enableformation of the desired frost image.

2. A method for varying the gamma characteristics of a frostablephotoreceptor member comprising initially charging said member to apotential V0 and, subsequent to exposure to an image, recharging saidmember to a potential VR where the ratio of the absolute values V0 to VRis less than unity.

3. A method for changing the response characteristics of a frostablephotoreceptor member from an initial characteristic of threshold andmaximum frost densities which correspond to first and second levels ofexposure illumination, respectively wherein the member is charged to aninitial potential, exposed suitably at one of said exposure illuminationlevels, recharged to an initial recharge potential, and softened,comprising the steps of:

(a) charging said member to said initial potential;

(b) exposing said member to a light pattern;

(c) recharging said member to a recharge potential, the absolute valueof which is greater than the absolute value of said in initialpotential, and

(d) softening said member to deform an exposed surface thereof inaccordance with said light pattern.

References Cited UNITED STATES PATENTS 7/1965 Mihajlou et al 96-1.17/1965 Gunther et a1. 96--1.1

U.S. Cl. X.R.

